The term “airfoil” broadly describes several species of elongate machined parts, such as blades, vanes and nozzles. As used in this application the term “workpiece” is used to describe any object to be shaped while centered on a machine tool, and the term “airfoil” refers to machined parts, such as blades, vanes and the like that are incorporated into fanjet engines, gas turbines and the like. This application describes the invention in the particular environment of the manufacture of airfoils. Such airfoils must be precisely machined to a predetermined aerodynamic shape having critical tolerances that must be duplicated from airfoil to airfoil. At the high rotational speeds of operation found in turbine engines, any excessive deviation from the required shape can cause high rates of wear, vibration and even catastrophic failure. Care is therefore required in mounting, for example, a blank in a machine tool, or fixture, where the blank will be machined into an airfoil such as a jet engine blade or vane.
As disclosed and explained in this application by way of example, an improved apparatus and method is provided for centering a workpiece such as a fan rotor blade in a fixture for a dovetail machining operation. A fan rotor blade includes a tip, an opposed root and a midspan region extending between the tip and root, and which defines the major longitudinal extent of the blade. A number of details must be machined into the workpiece during formation of the blade. These include a plurality of dovetails formed in the root by which the blade is mounted and secured to the rotor disk of the engine.
Reference is made to applicant's U.S. Pat. No. 6,068,541 that discloses the general machine environment within which the present invention is practiced.
Prior art devices and methods for centering workpieces for a machining operation include an operator-manipulated hand screw used to tighten a centering pin against a tip-locating block temporarily mounted on the tip of the workpiece to be shaped into the airfoil. This centering method introduces an unavoidable degree of variation into the workpiece loading process because no two operators will have the same “feel” for the proper clamping technique. Typically, prior art practices result in a varying amount of torque being applied to the hand screw, and thus a varying amount of tension directed along the stack axis of the workpiece during machining. Insufficient tension on the workpiece can result in movement during the process of clamping the workpiece into a stationary position and a mislocation relative to the workpiece-engaging milling tools. Similarly, excessive tension can result in distortion of the workpiece, deviation from the required stack axis location and misalignment of subsequently-machined features on the workpiece.